opp: Don't use IS_ERR on invalid supplies

_set_opp_custom() receives a set of OPP supplies as its arguments and
the caller of it passes NULL when the supplies are not valid. But
_set_opp_custom(), by mistake, checks for error by performing
IS_ERR(old_supply) on it which will always evaluate to false.

The problem was spotted during of testing of upcoming update for the
NVIDIA Tegra CPUFreq driver.

Cc: stable <stable@vger.kernel.org>
Fixes: 7e535993 ("OPP: Separate out custom OPP handler specific code")
Reported-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
[ Viresh: Massaged changelog ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>